Effect studies using environmentally relevant microplastic exposure levels, under various warming scenarios, are relevant to ecological risk assessments and environmental management. Yet scenario of exposure in laboratory studies are often not aligned with field conditions. To assess the concurrent effects that microplastics and temperature exposure have on the health and fitness of juvenile Acanthochromis polyacanthus, this study, conducted during the austral winter, used environmentally relevant predicted concentrations of polyester (PET) fibres (0, 1.1 and 11 PET L-1) and predicted elevated winter seawater temperatures experienced on the central Great Barrier Reef (GBR) (2020 23.7, 2050 25.2 and 2100 26.7˚C). An 8-week chronic microplastic exposure was simulated using a custom-built automated dosing system, developed specifically for this experiment. During this period, the system was maintained at seawater temperatures representing a 5-year averaged July daily temperature based on observations from Davis Reef, central GBR (ambient), or predicted future temperatures. Multiple physiological endpoints were examined to comprehensively assess effects of concurrently increasing microplastics and temperature on key molecular and phenotypic health and fitness parameters such as cortisol, total lipids, growth and condition factor were measured under the conditions evaluated. Microplastic uptake by A. polyacanthus only occurred via ingestion and was influenced solely by microplastic concentration. Increased daily winter temperature correlated to elevated levels of cortisol, but this was significantly decreased by concomitant exposure to the highest tested PET concentration. However, no measurable effects on growth, body condition and total lipid content were observed, suggesting that the impact of continued unabated microplastic contamination whilst mean GBR winter seawater temperatures increase, will be subtle.